Conscious rats maintained for three weeks at PB 370-380 Torr were studied in a chamber where PIO2 was maintained at 68-70 Torr at ambient barometric pressure (740-750 Torr). Controls were pair-fed rats maintained at ambient barometric pressure and studied at ambient PIO2 for 4 h. Steady-state intracellular pH (pHi) of left and right ventricle, and of tibialis anterior, quadriceps and diaphragm was determined from the distribution of 5,5-dimethyl-2,4-oxazolidinedione (DMO). Apparent non-bicarbonate buffer value (beta app) was calculated as the ratio of the change in HCO3- concentration to the change in pH elicited by the increase in PCO2. beta app of plasma, tibialis anterior, quadriceps and diaphragm was approximately 2, 3, 6 and 12 times higher, respectively, in hypoxic than in normoxic rats. Neither left nor right ventricular beta app was significantly changed by prolonged hypoxia. In the hypoxic animals, bilateral nephrectomy abolished the increase in beta app of plasma, tibialis anterior and quadriceps, and moderated the increase in beta app of diaphragm. No significant effect of nephrectomy was observed in beta app of either left or right ventricle. The results indicate that in the skeletal muscles studied under conditions of an acid load in the form of increased PCO2, intracellular pH is better regulated in hypoxic than in normoxic rats. The effects of nephrectomy suggest that this is due, at least in part, to a more effective renal compensation in hypoxic than in normoxic rats. Prolonged hypoxia, on the other hand, does not affect the cell pH regulation of right or left ventricle.